Semiconductor devices are devices that employ semiconductor materials, which are solid materials that exhibit an electrical conductivity lying between that of a conductor and that of an insulator. Semiconductor devices include, for example, diodes (e.g., light emitting diodes (LEDs)), photovoltaic devices, sensors, solid-state lasers, and integrated circuits (e.g., memory modules and microprocessors).
Photovoltaic devices are semiconductor devices that convert photons (e.g., light) into electricity. For example, solar panels include photovoltaic devices that convert sunlight (i.e., photons originating from the sun) into electricity. Due to the ever-increasing demand for renewable energy sources, the market for photovoltaic devices has experienced an average annual growth rate of about twenty-five percent (25%) over the previous decade.
Extensive research and development has resulted in photovoltaic materials and devices that are cheaper and more efficient. The cost of power produced by photovoltaic devices has decreased significantly over the past several decades, but must be further reduced to become competitive with alternative power sources, such as coal.
A majority of photovoltaic devices that are commercially available at the present time comprise photodiodes formed in silicon substrates. The performance of such silicon-based photovoltaic devices, is however, inherently limited by physical and chemical properties of silicon. New photovoltaic devices have been created that are based on light-absorbing materials (which may be either organic or inorganic) other than silicon. The number of non-silicon-based photovoltaic devices has steadily increased over the previous two (2) decades and currently accounts for more than ten percent (10%) of the solar energy market. Non-silicon photovoltaic devices are expected to eventually replace a large portion of the market for silicon-based photovoltaic devices and to expand the solar energy market itself due to their material properties and efficient power generating ability. In order for solar power to be economically competitive with alternative fossil fuel power sources at their current prices, photovoltaic devices based on photoactive materials other than silicon must be improved and further developed.
Materials other than silicon that can be employed in photovoltaic devices include, for example, germanium (Ge), chalcopyrites (e.g., CuInS2, CuGaS2, and CuInSe2), chalcogenides [Cu(InxGa1-x)(SexS1-x)2], cadmium telluride (CdTe), gallium arsenide (GaAs), organic polymers (e.g., polyphenylene vinylene, copper phthalocyanine, fullerenes), and light absorbing dyes (e.g., ruthenium-centered metalorganic dyes). Photovoltaic devices based on such materials have demonstrated greater photon conversion efficiencies than those exhibited by silicon-based devices. Furthermore, some non-silicon photovoltaic devices are capable of capturing a broader range of electromagnetic radiation than silicon-based devices, and as such, may be more efficient in producing electrical power from solar energy than are silicon-based devices.
Non-silicon photovoltaic devices may comprise thin films of photoactive materials, which may comprise polycrystalline materials or nanoparticles. The thin films of photoactive materials may be formed on flexible substrates such as polyethylene terephthalate (such as that sold under the trade name MYLAR®), which allows for a broad range of new configurations, designs, and applications for photovoltaic devices that were previously unavailable to silicon-based devices. Furthermore, thin film designs may use less than one percent (1%) of the raw materials used in conventional silicon-based devices, and therefore, may cost much less than silicon-based devices in terms of basic raw materials.
Manufacturing processes for thin films of photoactive materials include electroplating techniques, vapor deposition, flash evaporation, and evaporation from binary compounds, spray pyrolysis, and radiofrequency or ion beam sputtering of polycrystalline materials. Unfortunately, a majority of the costs associated in producing thin film photovoltaic devices are incurred in the thin film manufacturing techniques. In addition to being costly, existing thin film manufacturing processes tend to introduce a high number of defects into the films, which can result in an entire batch of material to be rendered inoperable. The next generation of photovoltaic devices would significantly impact the solar energy market if more efficient thin film manufacturing techniques and improved materials could be developed to overcome limitations of conventional processes and materials.